Storyline 1: Can't insert a shape on my slide

I have been working on this course for weeks, and suddenly today I am unable to insert a shape of any kind on any slide. When I select from Insert > Shape and then drag on the slide, all I see is a dotted outline with sizing and rotating handles. I closed the file and closed Storyline, then tried again. Same result.

I tried opening another course I recently completed, and I was able to insert various shapes - so it seems to be just this course.

I can work with yesterday's copy of the file and just lose the work I did this morning, but I'm curious as to what may have caused this issue. Thanks.

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Whoa! I just tried yesterday's and Friday's versions of the course, and the same thing is happening. At some point it must have gotten corrupted, but I don't know if it can be fixed or how to fix it.